1. Field of the Invention
This invention relates to methods and apparatus for the removal of a metallic film from an insulative substrate. In particular, the invention is directed to the selective removal of portions of the metallic film using a beam of coherent light energy.
2. Description of the Prior Art
It is well known to selectively remove metallic coatings from an insulative substrate using a coherent laser beam; for example, see U.S. Pat. No. 3,911,444 to Lou et al. which issued on Oct. 7, 1975. This patent discloses the use of a laser beam to scan a metallized substrate. The laser beam is modulated to vaporize predetermined portions of the metallic coating. The remaining metal defines an electrical connecting pattern for printed wiring boards, or the like.
In addition to the modulation of the laser beam to remove predetermined portions of the metallic film, it is known to interpose an apertured mask between the laser source and the metallized substrate as disclosed in copending U.S. patent application Ser. No. 754,293 to Koo et al., filed of even date herewith and assigned to the same assignee as the instant application. In that application, the apertured mask is described as being placed proximate the metallized substrate and the surface of the metallized substrate raster scanned through the mask. Portions of the substrate metallization are selectively removed where the coherent laser beam passes through the mask apertures to form the desired metallized pattern on the substrate. A transparent material is placed in contact with the mask to protect the mask and receive the metal debris removed from the metallized substrate. By maintaining the apertured mask and metallized substrate in spaced relationship, the metal removed from the substrate does not redeposit on the substrate. Such redeposition of material on the substrate can result in undesirable low resistance paths and shorts between the electrical connecting paths after subsequent electroplating operations.
The apertured mask may be formed on a rigid, clear glass plate by selectively depositing areas of opaque material thereon which represent the circuit pattern to be formed on the substrate. The glass plate with the apertured mask thereon can be a quarter of an inch thick or more and presents a substantially planar surface. However, the metallized substrate is usually a sixteenth of an inch or less in thickness, may have a surface area of over 400 square inches, and cannot readily be held in a planar orientation. Accordingly, it is not possible to place the apertured mask and metallized substrate in spaced planar parallel relationship for there will always be substantial variation in the distance therebetween due to the unevenness of the metallized substrate.
When the mask-to-substrate distance varies in this manner, the definition of the resulting circuit pattern is adversely affected due to diffraction of the coherent light beam. Additionally, such diffraction can result in an undesirable change in the intensity of light impinging on a given area of the metallized substrate, resulting in damage to the substrate or in a little or no removal of the metal which is manifested as shorts in the connecting paths printed wiring board.